In laboratory and other applications, automated liquid handlers that transport liquid samples are used in a variety of laboratory procedures. One example of an automated liquid handler is disclosed in U.S. Pat. No. 5,988,236 (“the '236 patent”) assigned to the assignee of the present application and incorporated herein by reference. The liquid handler of the '236 patent has a work bed that supports an array of sample containers, with multiple probes supported on an automated mover over the work bed. The automated mover is capable of moving the probes into alignment with one or more sample containers on the work bed to carry out liquid handling operations. Another example of a liquid handler can be found in U.S. Pat. No. 4,422,151, incorporated herein by reference.
Liquid chromatography, including high-performance liquid chromatography (HPLC), is one example of an application in which automated liquid handlers are used. Liquid chromatography is useful in characterizing a sample through separation of its components by flow through a chromatographic column, followed by detection of the separated components with a flow-through detector. Some HPLC systems include an automated liquid handler to load samples. In these systems, the liquid handler moves probes to load samples from sample containers and then inject the samples into an injection port. A metal needle may be attached to the probe to facilitate extraction of the sample from the container and injection of the sample into the injection port.
Although HPLC and other chemical test systems that include automated liquid handling are known, many long standing problems remain unresolved. As an example of an unsolved problem in liquid handling, carryover from one sample to subsequent samples can cause test contamination and inaccuracy when using many liquid handlers.
Carryover occurs when residue of a first sample remains in or on the probe or in the injection port and is then mixed with a subsequent sample. To reduce carryover, automated liquid handlers in chromatography and other test systems typically perform two solvent flushes between samples. A first flush is performed with the probe in the injection port to flush the port and the lines connected thereto. The probe and needle are then removed from the injection port, moved to a flushing position, and flushed a second time. Even with flushing, however, some carryover may occur. Additional flushing reduces carryover but slows processing and adds cost.
Another example of a problem connected with automated liquid handling methods in chromatography includes the presence of dead space associated with the samples. Dead space is an artifact of the type of sample injection system. Generally, samples are injected using a pressure differential that may include a driving force of air or inert gas and/or a drawing force of vacuum. In chromatography, the amount of sample that can be injected, called the test sample volume capacity also may be referred to as the test loop volume. Concerning the loop volume, known injection methods generally using known injection ports, probes and needles can result in considerable foreign material such as air being present in the test loop volume. For instance, in known injection methods, if the flow through a loading needle is too slow, or if a good seal is not provided between a probe needle and the injection port, air or other foreign material may be loaded on the chromatography instrument. To minimize the risk of not enough sample and too much foreign material, an excess of sample is typically loaded in the probe and injection port. In order to successfully load the correct amount of sample, known automated loading methods may require about four times or more of test loop volume to ensure that no inert gas or void space is injected. This amount of excess volume adds expense and time to testing, not to mention that the excess wastes valuable sample.
Still another known problem in automated handling for chemical analysis relates to the lack of reproducibility of volumetric measurements. An advantage to accurate volumetric measurements includes desirably minimizing specific variations in sample volume from test to test. However, methods to determine volumetric accuracy using known probes and attached needles is limited.
This invention also solves an additional problem found with many conventional HPLC systems. In many liquid handling applications including HPLC, bio-compatible components are required. Although some systems use pumping and injection valves made from biocompatible PEEK or biocompatible titanium, there is ultimately still a non-compatible component (often stainless steel) in the injection needle. In order to mask the non-compatible element, the injection needles may be either coated or made from titanium to reduce the metallic component. However, these modifications fail to reduce carry over and when coated injection needles are used, problems can arise as the coating wears.